1. Field
The invention is directed toward electronic filters and methods of their use. Specifically, the invention is directed toward reflectionless electronic filters and methods of their use.
2. Background
Virtually all electronic systems use some kind of filtering to reject unwanted frequency components. In most conventional filters, the rejected signals are bounced back to the source, eventually dissipating in the generator itself, or in the interconnecting wires/transmission lines, or by being radiated into the instrument housing. This manner of rejecting unwanted signals can sometimes lead to harmful interactions with other components in the system, either by spurious mixing in non-linear devices, unintentional re-biasing of sensitive active components, or cross-talk between various signal paths. A solution was sought for a filter that would absorb these unwanted signals before they could compromise performance. This led to a novel absorptive filter topology, which is described in U.S. Pat. No. 8,392,495 to Morgan, the entirety of which is incorporated by reference herein. FIG. 1 depicts an example of a low-pass reflectionless filter of the prior art. The absorptive filter solved many problems encountered with conventional filters, such as the sensitivity of mixers to poor out-of-band terminations, detrimental and difficult-to-predict non-linear effects from reactive harmonic loading, leakage or cross-talk due to trapped energy between the filter and other poorly-matched components, and numerous other problems associated with out-of-band impedance matching. It also realized superior performance and manufacturability when compared to other approaches to absorptive filters, such as terminated diplexers and directional filter structures employing quadrature hybrids.
Despite these benefits, however, some applications require a sharper cutoff response than the previously demonstrated reflectionless filters can provide. Recent efforts to address this issue have yielded an enhanced version of the structure which can have a much sharper cutoff response and thus yield greater selectivity in those applications where it is required, while maintaining all the benefits of the original reflectionless filter topology.